U.S. stocks are flirting with all-time highs, climbing to heights not seen since before the financial crisis. Both the Dow and the S&amp;P 500 have risen to their highest levels since October 2007. But stock prices cannot go up forever, and some analysts warn that the bull market is nearing an end, just as investors are returning.

Facebook's fourth-quarter financial results surpassed Wall Street's expectations, but its stock is dipping lower in extended trading. The social networking company earned $64 million, or 3 cents per share, in the October-December period.